MAESTRO: fix(db): prevent FK constraint failures on worker restart

Cherry-picked source changes from PR #889 by @Et9797. Fixes #846.

Key changes:
- Add ensureMemorySessionIdRegistered() guard in SessionStore.ts
- Add ON UPDATE CASCADE migration (schema v21) for observations and session_summaries FK constraints
- Change message queue from claim-and-delete to claim-confirm pattern (PendingMessageStore.ts)
- Add spawn deduplication and unrecoverable error detection in SessionRoutes.ts and worker-service.ts
- Add forceInit flag to SDKAgent for stale session recovery

Build artifacts skipped (pre-existing dompurify dep issue). Path fixes (HealthMonitor.ts, worker-utils.ts)
already merged via PR #634.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/worker/GeminiAgent.ts

@@ -186,6 +186,10 @@ export class GeminiAgent {
       let lastCwd: string | undefined;
 
       for await (const message of this.sessionManager.getMessageIterator(session.sessionDbId)) {
+        // CLAIM-CONFIRM: Track message ID for confirmProcessed() after successful storage
+        // The message is now in 'processing' status in DB until ResponseProcessor calls confirmProcessed()
+        session.processingMessageIds.push(message._persistentId);
+
         // Capture cwd from each message for worktree support
         if (message.cwd) {
           lastCwd = message.cwd;